1. Field of the Invention
The invention relates to filters and, more particularly, to voltage controlled resonant-type filters used in electronic music synthesis.
2. Prior Art
Filters are commonly used in musical signal processing to controllably modify selected characteristics of a signal being filtered. Typically, the cut-off frequency of the filter is made adjustable in response to a control voltage which sets the cut-off frequency of the filter and thereby enables the generation of varying output signals from a standard input signal such as a square wave, triangular wave, etc. Such filters typically use conventional (voltage responsive) operational amplifiers connected in varying configurations such as the integrating configuration, the amplifying configuration, etc. to perform the requisite wave shaping. An especially useful filter of this type is described in U.S. Pat. No. 3,805,091, issued Apr. 16, 1974 to Dennis P. Colin and assigned to the assignee of the present invention. The present invention constitutes a further development of, and improvement over, that filter.
Resonant filters utilize controlled amounts of positive feedback within the filter to provide resonance at a selected frequency. For example, in music synthesizers, pleasing musical effects can be obtained by applying a square-wave to a filter and utilizing a controlled amount of positive feedback within the filter to provide resonance at the fundamental frequency of the wave. An example of a resonant filter is shown in U.S. Pat. No. 3,924,199 issued Dec. 2, 1975 to Alan R. Pearlman and assigned to the assignee of the present invention.
Because of the potential instability problems imposed by positive feedback, it is necessary to limit the audio input signal, the feedback signal, or both. Typically, the limiting is accomplished by utilizing diode clippers. Such clippers are characterized by abrupt transitions into conduction, and thus do not allow smooth control of resonance. Because of this, higher order harmonics are frequently created which distort the audio signal and may also cause the resonant filter to lock onto one or more of these harmonics uncontrollably. Further, they are susceptible to non-symmetric clipping of the output signal, and have an undesirable signal variation with temperature.